1. Field of the Invention
This invention relates to computer systems and more specifically to a computer display controller especially suitable for use in portable computers having a flat panel-type display.
2. Description of the Prior Art
Portable computers, i.e. notebook computers and other types of portable computers, typically include an LCD-type display, also referred to herein as a flat panel display. This is in contrast to the CRT (cathode ray tube) type display typically used for larger desk top computers. These flat panel displays come in various types, including TFT (thin film transistor) active matrix type and STN (super twisted nematic) passive matrix type. Both of these support color imagery. Flat panel displays variously have 8 bit, 9 bit, 16 bit, 18 bit, 12 bit, or 24 bit per pixel etc. interfaces and have various screen resolutions in terms of numbers of pixels and characters, as well as various refresh rates. Passive matrix type (STN) flat panel displays are dual scan, i.e. they typically include two displays which are physically vertically contiguous to each other, one being the upper panel and the second being the lower panel, each requiring its own driver circuitry. A controller drives these two half-displays and provides all parameters in terms of screen resolution, bits per pixel (number of colors), refresh rate and horizontal scan rate. A computer display/graphics subsystem includes a controller and a frame buffer where the image is stored and manipulated, independent of the computer's main memory.
The other type of computer display, which is the CRT-type display, also has its own parameters in terms of screen resolution, number of colors/shades, horizontal scan rate, and refresh rate. Such CRT-type displays include a CRT display and a television-type monitor.
For a portable computer, it is desirable in addition to the built-in flat panel display to connect an external CRT-type display or television. This is typically done when at one's office to use two displays, or for instance during a presentation when the presenter is using his portable computer to drive a video projector or monitor for viewing by an audience, while the presenter looks at the computer flat panel display and thereby can face the audience.
In these applications either identical or different images are presented on the two displays, i.e. the flat panel display and the CRT. A prior art product is the Apple Powerbook.TM. portable computer which in order to support two such displays, projecting different information simultaneously and independently, includes two separate display controller systems, one driving the flat panel display and the second driving the CRT display. Thus there are two display controllers and two frame buffers, i.e. two complete display subsystems, one dedicated to each display. This is expensive in terms of circuitry and also consumes more power than is desired. Other prior art portable computers, while they can simultaneously provide images both on the built-in flat panel display and on the connected external CRT display, can display only a single identical image on both, since they include only a single display controller system. Thus the prior art solution to providing a dual image mode is a `brute force" approach requiring expensive and power consuming circuitry as in the Apple Powerbook product. Dual image mode refers to the capability where the image on each display is independent and can be separately manipulated.